Memory cell programming with a programming pulse having plurality of different voltage levels

ABSTRACT

Methods of operating a memory include applying a programming pulse having a particular voltage level to a selected access line connected to a plurality of memory cells selected for programming during a programming operation, concurrently enabling for programming each memory cell of the plurality of memory cells selected for programming while applying the programming pulse, applying a subsequent programming pulse having a plurality of different voltage levels to the selected access line, and, for each group of memory cells of a plurality of groups of memory cells of the plurality of memory cells selected for programming, enabling that group of memory cells for programming while the subsequent programming pulse has a corresponding voltage level of the plurality of different voltage levels.

RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.15/665,474, titled “MEMORY CELL PROGRAMMING,” filed Aug. 1, 2017, nowU.S. Pat. No. 10,037,806, issued on Jul. 31, 2018, which is a Divisionalof U.S. patent application Ser. No. 15/072,954, titled “MEMORY CELLPROGRAMMING,” filed Mar. 17, 2016, now U.S. Pat. No. 9,767,909, issuedon Sep. 19, 2017, which are commonly assigned and incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates generally to memory and, in particular,in one or more embodiments, the present disclosure relates to memorycell programming.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuit devices in computers or other electronic devices.There are many different types of memory including random-access memory(RAM), read only memory (ROM), dynamic random access memory (DRAM),synchronous dynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Changes in threshold voltage (Vt) of the memory cells, throughprogramming (which is often referred to as writing) of charge storagestructures (e.g., floating gates or charge traps) or other physicalphenomena (e.g., phase change or polarization), determine the data state(e.g., data value) of each memory cell. Common uses for flash memoryinclude personal computers, personal digital assistants (PDAs), digitalcameras, digital media players, cellular telephones, solid state drivesand removable memory modules, and the uses are growing.

Programming in memories is typically accomplished by applying aplurality of programming pulses, separated by verify pulses, to programeach memory cell of a selected group of memory cells to a respectiveintended data state (which may be an interim or final data state). Withsuch a scheme, the programming pulses are applied to access lines, suchas those typically referred to as word lines, for selected memory cells.After each programming pulse, one or more verify pulses are used toverify the programming of the selected memory cells. Current programmingtypically uses many programming pulses in an incremental step pulseprogramming scheme, where each programming pulse is a single pulse thatmoves the memory cell threshold voltage by some amount. Before eachprogramming pulse, word lines may be precharged, and after eachprogramming pulse, the word lines may be discharged. This can lead tohigh power consumption.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternative methods of operating memory, and apparatus to perform suchmethods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a memory in communication with aprocessor as part of an electronic system, according to an embodiment.

FIG. 2 is a schematic of a portion of an array of memory cells as couldbe used in a memory of the type described with reference to FIG. 1.

FIG. 3 illustrates an example of threshold voltage ranges for apopulation of multi-level memory cells.

FIG. 4 depicts a shift in threshold voltage distribution followingapplication of a programming pulse to a number of memory cells accordingto an embodiment.

FIG. 5 depicts a flowchart of a method of operating a memory accordingto an embodiment.

FIGS. 6A-6D depict a method of determining VgVt for memory cells of adistribution of threshold voltages and programming those memory cells totheir intended data state according to an embodiment.

FIG. 7A depicts a flowchart of a method of operating a memory accordingto an embodiment.

FIG. 7B depicts a portion of a flowchart of a variation of the method ofFIG. 7A according to an embodiment.

FIG. 8 conceptually depicts waveforms of voltage levels for variousnodes in performing a method such as described with reference to FIG. 7Aaccording to an embodiment.

FIGS. 9A-9B depict an alternate method of determining VgVt for memorycells of a distribution of threshold voltages including negativethreshold voltages according to an embodiment.

FIG. 10 depicts a method of reducing a width of a distribution ofthreshold voltages for use with various embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown, byway of illustration, specific embodiments. In the drawings, likereference numerals describe substantially similar components throughoutthe several views. Other embodiments may be utilized and structural,logical and electrical changes may be made without departing from thescope of the present disclosure. The following detailed description is,therefore, not to be taken in a limiting sense.

FIG. 1 is a simplified block diagram of a first apparatus, in the formof a memory (e.g., memory device) 100, in communication with a secondapparatus, in the form of a processor 130, as part of a third apparatus,in the form of an electronic system, according to an embodiment. Someexamples of electronic systems include personal computers, personaldigital assistants (PDAs), digital cameras, digital media players,digital recorders, games, appliances, vehicles, wireless devices,cellular telephones and the like. The processor 130, e.g., a controllerexternal to the memory device 100, may be a memory controller or otherexternal host device.

Memory device 100 includes an array of memory cells 104 logicallyarranged in rows and columns. Memory cells of a logical row aretypically connected to the same access line (commonly referred to as aword line) while memory cells of a logical column are typicallyselectively connected to the same data line (commonly referred to as abit line). A single access line may be associated with more than onelogical row of memory cells and a single data line may be associatedwith more than one logical column. A block of memory cells may includethose memory cells that are configured to be erased together, such asall memory cells connected to word lines 202 ₀-202 _(N) (e.g., all NANDstrings 206 sharing common word lines 202). Memory cells (not shown inFIG. 1) of at least a portion of array of memory cells 104 are capableof being programmed to one of at least two data states.

A row decode circuitry 108 and a column decode circuitry 110 areprovided to decode address signals. Address signals are received anddecoded to access the array of memory cells 104. Memory device 100 alsoincludes input/output (I/O) control circuitry 112 to manage input ofcommands, addresses and data to the memory device 100 as well as outputof data and status information from the memory device 100. An addressregister 114 is in communication with I/O control circuitry 112 and rowdecode circuitry 108 and column decode circuitry 110 to latch theaddress signals prior to decoding. A command register 124 is incommunication with I/O control circuitry 112 and control logic 116 tolatch incoming commands.

An internal controller (e.g., control logic 116) controls access to thearray of memory cells 104 in response to the commands and generatesstatus information for the external processor 130, i.e., control logic116 is configured to perform access operations (e.g., programmingoperations) in accordance with embodiments described herein. The controllogic 116 is in communication with row decode circuitry 108 and columndecode circuitry 110 to control the row decode circuitry 108 and columndecode circuitry 110 in response to the addresses.

Control logic 116 is also in communication with a cache register 118 anddata register 120. Cache register 118 latches data, either incoming oroutgoing, as directed by control logic 116 to temporarily store datawhile the array of memory cells 104 is busy writing or reading,respectively, other data. During a programming operation (e.g., oftenreferred to as a write operation), data is passed from the cacheregister 118 to the data register 120 for transfer to the array ofmemory cells 104; then new data is latched in the cache register 118from the I/O control circuitry 112. During a read operation, data ispassed from the cache register 118 to the I/O control circuitry 112 foroutput to the external processor 130; then new data is passed from thedata register 120 to the cache register 118. A status register 122 is incommunication with I/O control circuitry 112 and control logic 116 tolatch the status information for output to the processor 130.

Memory device 100 receives control signals at control logic 116 fromprocessor 130 over a control link 132. The control signals may includeat least a chip enable CE #, a command latch enable CLE, an addresslatch enable ALE, and a write enable WE #. Additional control signals(not shown) may be further received over control link 132 depending uponthe nature of the memory device 100. Memory device 100 receives commandsignals (which represent commands), address signals (which representaddresses), and data signals (which represent data) from processor 130over a multiplexed input/output (I/O) bus 134 and outputs data toprocessor 130 over I/O bus 134.

For example, the commands are received over input/output (I/O) pins[7:0] of I/O bus 134 at I/O control circuitry 112 and are written intocommand register 124. The addresses are received over input/output (I/O)pins [7:0] of bus 134 at I/O control circuitry 112 and are written intoaddress register 114. The data are received over input/output (I/O) pins[7:0] for an 8-bit device or input/output (I/O) pins [15:0] for a 16-bitdevice at I/O control circuitry 112 and are written into cache register118. The data are subsequently written into data register 120 forprogramming the array of memory cells 104. For another embodiment, cacheregister 118 may be omitted, and the data are written directly into dataregister 120. Data are also output over input/output (I/O) pins [7:0]for an 8-bit device or input/output (I/O) pins [15:0] for a 16-bitdevice.

It will be appreciated by those skilled in the art that additionalcircuitry and signals can be provided, and that the memory device 100 ofFIG. 1 has been simplified. It should be recognized that thefunctionality of the various block components described with referenceto FIG. 1 may not necessarily be segregated to distinct components orcomponent portions of an integrated circuit device. For example, asingle component or component portion of an integrated circuit devicecould be adapted to perform the functionality of more than one blockcomponent of FIG. 1. Alternatively, one or more components or componentportions of an integrated circuit device could be combined to performthe functionality of a single block component of FIG. 1.

Additionally, while specific I/O pins are described in accordance withpopular conventions for receipt and output of the various signals, it isnoted that other combinations or numbers of I/O pins may be used in thevarious embodiments.

FIG. 2 is a schematic of a NAND memory array 200, e.g., as a portion ofarray of memory cells 104. Memory array 200 includes access lines, suchas word lines 202 ₀ to 202 _(N), and data lines, such as bit lines 204 ₀to 204 _(M). The word lines 202 may be connected to global access lines(e.g., global word lines), not shown in FIG. 2, in a many-to-onerelationship. For some embodiments, memory array 200 may be formed overa semiconductor that, for example, may be conductively doped to have aconductivity type, such as a p-type conductivity, e.g., to form ap-well, or an n-type conductivity, e.g., to form an n-well.

Memory array 200 might be arranged in rows (each corresponding to a wordline 202) and columns (each corresponding to a bit line 204). Eachcolumn may include a string of series-connected memory cells, such asone of NAND strings 206 ₀ to 206 _(M). Each NAND string 206 might beconnected (e.g., selectively connected) to a common source 216 and mightinclude memory cells 208 ₀ to 208 _(N). The memory cells 208 representnon-volatile memory cells for storage of data. The memory cells 208 ofeach NAND string 206 might be connected in series between a selecttransistor 210 (e.g., a field-effect transistor), such as one of theselect transistors 210 ₀ to 210 _(M) (e.g., that may be source selecttransistors, commonly referred to as select gate source), and a selecttransistor 212 (e.g., a field-effect transistor), such as one of theselect transistors 212 ₀ to 212 _(M) (e.g., that may be drain selecttransistors, commonly referred to as select gate drain). Selecttransistors 210 ₀ to 210 _(M) might be commonly connected to a selectline 214, such as a source select line, and select transistors 212 ₀ to212 _(M) might be commonly connected to a select line 215, such as adrain select line.

A source of each select transistor 210 might be connected to commonsource 216. The drain of each select transistor 210 might be connectedto a memory cell 208 ₀ of the corresponding NAND string 206. Forexample, the drain of select transistor 210 ₀ might be connected tomemory cell 208 ₀ of the corresponding NAND string 206 ₀. Therefore,each select transistor 210 might be configured to selectively connect acorresponding NAND string 206 to common source 216. A control gate ofeach select transistor 210 might be connected to select line 214.

The drain of each select transistor 212 might be connected to the bitline 204 for the corresponding NAND string 206. For example, the drainof select transistor 212 ₀ might be connected to the bit line 204 ₀ forthe corresponding NAND string 206 ₀. The source of each selecttransistor 212 might be connected to a memory cell 208 _(N) of thecorresponding NAND string 206. For example, the source of selecttransistor 212 ₀ might be connected to memory cell 208 _(N) of thecorresponding NAND string 206 ₀. Therefore, each select transistor 212might be configured to selectively connect a corresponding NAND string206 to a corresponding bit line 204. A control gate of each selecttransistor 212 might be connected to select line 215.

The memory array in FIG. 2 might be a quasi-two-dimensional memory arrayand might have a generally planar structure, e.g., where the commonsource 216, NAND strings 206 and bit lines 204 extend in substantiallyparallel planes. Alternatively, the memory array in FIG. 2 might be athree-dimensional memory array, e.g., where NAND strings 206 may extendsubstantially perpendicular to a plane containing the common source 216and to a plane containing the bit lines 204 that may be substantiallyparallel to the plane containing the common source 216.

Typical construction of memory cells 208 includes a data-storagestructure 234 (e.g., a floating gate, charge trap, etc.) that candetermine a data state of the memory cell (e.g., through changes inthreshold voltage), and a control gate 236, as shown in FIG. 2. In somecases, memory cells 208 may further have a defined source 230 and adefined drain 232. Memory cells 208 have their control gates 236connected to (and in some cases form) a word line 202.

A column of the memory cells 208 is a NAND string 206 or a plurality ofNAND strings 206 selectively connected to a given bit line 204. A row ofthe memory cells 208 are memory cells 208 commonly connected to a givenword line 202. A row of memory cells 208 can, but need not include allmemory cells 208 commonly connected to a given word line 202. Rows ofmemory cells 208 may often be divided into one or more groups ofphysical pages of memory cells 208, and physical pages of memory cells208 often include every other memory cell 208 commonly connected to agiven word line 202. For example, memory cells 208 commonly connected toword line 202 _(N) and selectively connected to even bit lines 204(e.g., bit lines 204 ₀, 204 ₂, 204 ₄, etc.) may be one physical page ofmemory cells 208 (e.g., even memory cells) while memory cells 208commonly connected to word line 202 _(N) and selectively connected toodd bit lines 204 (e.g., bit lines 204 ₁, 204 ₃, 204 ₅, etc.) may beanother physical page of memory cells 208 (e.g., odd memory cells).Although bit lines 204 ₃-204 ₅ are not expressly depicted in FIG. 2, itis apparent from the figure that the bit lines 204 of the array ofmemory cells 200 may be numbered consecutively from bit line 204 ₀ tobit line 204 _(M). Other groupings of memory cells 208 commonlyconnected to a given word line 202 may also define a physical page ofmemory cells 208. For certain memory devices, all memory cells commonlyconnected to a given word line might be deemed a physical page. Theportion of a physical page (which, in some embodiments, could still bethe entire row) that is read during a single read operation orprogrammed during a programming operation (e.g., an upper or lower pagememory cells) might be deemed a logical page.

Although the example of FIG. 2 is discussed in conjunction with NANDflash, the embodiments and concepts described herein are not limited toa particular array architecture or structure, and can include otherstructures (e.g., SONOS, phase change, ferroelectric, etc.) and otherarchitectures (e.g., AND arrays, NOR arrays, etc.).

Memory cells may be configured to operate as what are known in the artas single-level memory cells (SLC) or multi-level memory cells (MLC).SLC and MLC memory cells assign a data state (e.g., representing arespective value of one or more bits) to a specific range of thresholdvoltages (Vt) stored on the memory cells. Single level memory cells(SLC) permit the storage of a single binary digit (e.g., bit) of data oneach memory cell. Meanwhile, MLC technology permits the storage of morethan one binary digit per memory cell (e.g., two bits, three bits, fourbits, etc.), depending on the quantity of threshold voltage rangesassigned to the memory cell and the stability of the assigned thresholdvoltage ranges during the lifetime operation of the memory cell. By wayof example, one bit (e.g., 1 or 0) may be represented by two thresholdvoltage ranges, two bits by four ranges, three bits by eight ranges,etc. Non-binary numbers of threshold voltage ranges are also known,e.g., using two memory cells configured to operate with three datastates to collectively store three bits of information, or 1.5 bits permemory cell.

Programming typically involves applying one or more programming pulses(Vpgm) to a selected word line and thus to the control gates of the rowof memory cells coupled to the selected word line. Typical programmingpulses (Vpgm) may start at or near 15V and tend to increase in magnitudeduring each programming pulse application. While the program potential(e.g., voltage level of the programming pulse) is applied to theselected word line, an enable voltage, such as a ground potential (e.g.,0V), is applied to the channels of memory cells selected forprogramming, i.e., those memory cells for which the programmingoperation is intended to shift their data state to some higher level.This may result in a charge transfer from the channel to the chargestorage structures of these selected memory cells. For example, floatinggates are typically charged through direct injection or Fowler-Nordheimtunneling of electrons from the channel to the floating gate, resultingin a threshold voltage typically greater than zero in a programmedstate.

An inhibit voltage (e.g., Vcc) is typically applied to bit lines whichare selectively connected to a NAND string containing a memory cell thatis not selected for programming. In addition to bit lines selectivelyconnected to memory cells already at their intended data state, theseunselected bit lines may further include bit lines that are notaddressed by the programming operation. For example, a logical page ofdata may correspond to memory cells coupled to a particular word lineand selectively connected to some particular subset of the bit lines(e.g., every other bit line), such that the remaining subset of bitlines would be unselected for the programming operation and thusinhibited.

Between the application of one or more programming (e.g., Vpgm) pulses,a verify operation is typically performed to check each selected memorycell to determine if it has reached its intended data state. If aselected memory cell has reached its intended data state it is inhibitedfrom further programming if there remain other selected memory cellsstill requiring additional programming pulses to reach their intendeddata states. Following a verify operation, an additional programmingpulse (Vpgm) may be applied if there are memory cells that have notcompleted programming. This process of applying a programming pulsefollowed by performing a verify operation (e.g., a program-verify phaseof a programming operation) typically continues until all the selectedmemory cells have reached their intended data states. If a particularnumber of programming pulses (e.g., maximum number) have been appliedand one or more selected memory cells still have not completedprogramming, those memory cells might be marked as defective, forexample. Various embodiments seek to reduce a number of theseprogram-verify phases of a programming operation.

FIG. 3 illustrates an example of threshold voltage ranges for apopulation of a four-level (e.g., two-bit) MLC memory cells. Forexample, such a memory cell might be programmed to a threshold voltage(Vt) that falls within one of four different threshold voltage ranges301-304, each being used to represent a data state corresponding to abit pattern comprised of two bits. The threshold voltage range 301typically has a greater width than the remaining threshold voltageranges 302-304 as memory cells are generally all placed in the datastate corresponding to the threshold voltage range 301, then subsets ofthose memory cells are subsequently programmed to have thresholdvoltages in one of the threshold voltage ranges 302-304. As programmingoperations are generally more incrementally controlled than eraseoperations, these threshold voltage ranges 302-304 may tend to havetighter distributions.

The threshold voltage ranges 302-304 might each have a width 305, e.g.,a width of 750 mV. In addition, a dead space 306 (e.g., sometimesreferred to as a margin, and might be approximately 500 mV or greater)is typically maintained between adjacent threshold voltage ranges301-304 to keep the threshold voltage ranges from overlapping. As anexample, if the threshold voltage of a memory cell is within the firstof the four threshold voltage ranges 301, the memory cell in this caseis storing a logical ‘11’ data state and is typically referred to as theerased state of the memory cell. If the threshold voltage is within thesecond of the four threshold voltage ranges 302, the memory cell in thiscase is storing a logical ‘10’ data state. A threshold voltage in thethird threshold voltage range 303 would indicate that the memory cell inthis case is storing a logical ‘00’ data state. Finally, a thresholdvoltage residing in the fourth threshold voltage range 304 indicatesthat a logical ‘01’ data state is stored in the memory cell.

Various embodiments may utilize a determination of a relationshipbetween an applied voltage level (e.g., a gate voltage Vg) across amemory cell to its resulting threshold voltage as an indication of aprogramming voltage sufficient to program a memory cell, or group ofmemory cells, to a particular threshold voltage, or range of thresholdvoltages. This relationship may be referred to as VgVt and represents adifference between the applied voltage level across a memory cell andits resulting threshold voltage. For example, if a voltage level of 13volts is applied to a control gate of a memory cell whose body (e.g.,channel) is at a ground potential (e.g., 0 volts), and the resultingthreshold voltage is −0.5 volt, the VgVt for that memory cell is (13volts−0 volts)−(−0.5 volts)=13.5 volts.

It is expected that the VgVt relationship as a function of intendedthreshold voltage might be deemed to be linear within normal operationof a memory cell, and may have a positive slope, such that the VgVt at athreshold voltage of 0.5 volts for the same memory cell might beexpected to be greater than 13.5 volts. The VgVt relationship tothreshold voltage might be more accurately described by a polynomialequation. Regardless, the VgVt relationship for a particular memory as afunction of threshold voltage (e.g., the slope of a line, the constantsof a polynomial or other characterization) might be determinedempirically, based on knowledge of the structure and materials of thememory cells, or directly through experimentation. For example,programming pulses of various voltage levels can be applied to one ormore memory cells, and the resulting threshold voltages can bedetermined after each of these programming pulses. A composite functioncould be fitted from the individual responses of these memory cells. Afunction could be determined for a memory array as a whole, orindividual functions might be determined on some sub-portion of thememory array, e.g., by block of memory cells.

Once a VgVt value for a particular threshold voltage is determined for aparticular memory cell, the VgVt value for a different threshold voltagecan be calculated (e.g., corrected) by fitting the function to the knownvalue. To continue the foregoing example, if the relationship is deemedto be linear with a slope (e.g., ΔVgVt/ΔVt) of 0.2, and the VgVt at athreshold voltage of −0.5 volts is 13.5 volts, the VgVt at a thresholdvoltage of 0.5 volts might be expected to be 13.7 volts. Accordingly, aprogramming voltage (e.g., the programming voltage across the memorycell) of 14.2 volts might be expected to result in a threshold voltageof 0.5 volts for that memory cell. Where a non-linear function isutilized, the function could define the change in VgVt value from themeasured threshold voltage of a memory cell to the intended thresholdvoltage of that memory cell, and this ΔVgVt could be added to thedetermined VgVt of that memory cell at its measured threshold voltage inorder to calculate the VgVt value of that memory cell at its intendedthreshold voltage.

FIG. 4 depicts a shift in threshold voltage distribution followingapplication of a programming pulse (e.g., a discovery programming pulse)to a number of memory cells for use with various embodiments. Forexample, the memory cells of the distribution of threshold voltages 401might be in an initial data state, e.g., the erased data state. Thememory cells of the distribution of threshold voltages 401 might bethose memory cells selected for programming during a programmingoperation. If a programming pulse, e.g., having some positive voltagelevel relative to the channels of the selected memory cells, is appliedto the access line (e.g., word line) connected to the memory cells ofdistribution of threshold voltages 401, they might be expected toexperience an increase in threshold voltage, resulting in thedistribution of threshold voltages 403. Note that the distribution ofthreshold voltages 403 may be wider than distribution of thresholdvoltages 401.

The voltage level of this discovery programming pulse may be chosen tobe expected to produce the distribution of threshold voltages 403 tohave a range of threshold voltages that are each less than a range ofthreshold voltages corresponding to a data state (e.g., intended datastate) to which the memory cells of distribution of threshold voltages403 are to be programmed. The voltage level of this discoveryprogramming pulse might be determined empirically, based on knowledge ofthe structure and materials of the memory cells, or directly throughexperimentation. In one example, a programming pulse of 13 volts acrossa memory cell might produce a distribution of threshold voltages 403having a width of approximately 4 volts Some portion (e.g., less thanall or all) of the distribution of threshold voltages 403 may correspondto negative threshold voltages.

The voltage level of the discovery programming pulse may be furtherdependent upon the intended data state for a programming operation. Forexample, the discovery programming pulse might have a first voltagelevel for programming operations to bring memory cells to the data statecorresponding to the distribution of threshold voltages 302 of FIG. 3,the discovery programming pulse might have a second (e.g., higher)voltage level for programming operations to bring memory cells to thedata state corresponding to the distribution of threshold voltages 303of FIG. 3, and the discovery programming pulse might have a third (e.g.,higher) voltage level for programming operations to bring memory cellsto the data state corresponding to the distribution of thresholdvoltages 304 of FIG. 3.

The distribution of threshold voltages 403 resulting from the discoveryprogramming pulse provides information indicative of programmingvoltages sufficient to program memory cells of the distribution ofthreshold voltages 403 to a particular threshold voltage, or to programgroups of memory cells of the distribution of threshold voltages 403 toa particular range of threshold voltages, e.g., a range of thresholdvoltages corresponding to an intended data state. This may facilitateprogramming these memory cells to threshold voltages corresponding totheir intended data state with only a single additional programmingpulse. FIG. 5 depicts a flowchart of a method of operating a memoryaccording to an embodiment making use of this information.

At 540, an indication of a programming voltage sufficient to program agroup of memory cells to a particular range of threshold voltages isdetermined for each group of memory cells of a plurality of groups ofmemory cells. The groups of memory cells each correspond to somerespective range of threshold voltages of the distribution of thresholdvoltages 403, i.e., the memory cells of a particular group of memorycells each have a threshold voltage within its respective range ofthreshold voltages. A voltage level of a programming pulse resulting inthe distribution of threshold voltages 403, and a representativethreshold voltage of the particular group of memory cells, can providethe indication of the programming voltage sufficient to program theparticular group of memory cells to the particular range of thresholdvoltages. For example, a representative threshold voltage of a group ofmemory cells might be a lower value (e.g., lower limit) of itscorresponding range of threshold voltages, an upper value (e.g., upperlimit) of its corresponding range of threshold voltages, an average ofits corresponding range of threshold voltages, etc. Within theparticular group of memory cells, a particular voltage level of aprogramming pulse might be expected to shift each of the thresholdvoltages of the particular group of memory cells by an amount that mightbe deemed to be equal.

At 542, a stepped programming pulse is applied to a selected access line(e.g., word line) connected to each memory cell of the plurality ofgroups of memory cells. Note that the selected access line might beconnected to memory cells not included in the plurality of groups ofmemory cells, e.g., not selected for programming or addressed by theprogramming operation. The stepped programming pulse may have a seriesof successively different (e.g., lower or higher) voltage levels,including a respective voltage level for each group of memory cells ofthe plurality of groups of memory cells. At 544, when a voltage level ofthe stepped programming pulse corresponds to the respective indicationof the programming voltage sufficient to program a group of memory cellsto the particular range of threshold voltages, that group of memorycells is enabled for programming. This may be repeated for each group ofmemory cells of the plurality of groups of memory cells. Groups ofmemory cells not corresponding to the voltage level of the steppedprogramming pulse might be inhibited from programming.

FIGS. 6A-6D depict a method of determining VgVt for memory cells of adistribution of threshold voltages 403 and programming those memorycells to their intended data state according to an embodiment. It isnoted that FIGS. 6A-6D may depict theoretical expectations applied tonormal distributions. However, the concepts described can be appliedregardless of the shape of the distributions. Furthermore, while thediscussion of FIGS. 6A-6D utilizes eight threshold voltage ranges 611for simplicity, the number of threshold voltage ranges 611 may be feweror greater. It is noted, however, that increasing numbers of thresholdvoltage ranges may provide for more narrow distributions of thresholdvoltages after programming. For example, where the width of thedistribution of threshold voltages 403 is 4 volts, sixteen thresholdvoltages ranges 611 might facilitate a threshold voltage range afterprogramming of approximately 250 millivolts.

Following generation of the distribution of threshold voltages 403,i.e., following application of the discovery programming pulse to anaccess line selected for a programming operation, a plurality of groupsof memory cells are identified. These groups represent portions of thememory cells of the distribution of threshold voltages 403 havingthreshold voltages within respective threshold voltages ranges 611 a-611h as depicted in FIG. 6A. For example, those memory cells withindistribution of threshold voltages 403 and having a threshold voltagewithin the threshold voltage range 611 a might correspond to a firstgroup of memory cells, those memory cells within distribution ofthreshold voltages 403 and having a threshold voltage within thethreshold voltage range 611 b might correspond to a second group ofmemory cells, those memory cells within distribution of thresholdvoltages 403 and having a threshold voltage within the threshold voltagerange 611 c might correspond to a third group of memory cells, and soon.

Identification of the groups of memory cells can be accomplished bysensing (e.g., reading) the memory cells of the distribution ofthreshold voltages 403 at different read voltages. Sensing of memorycells generally involves applying a particular voltage level (e.g., readvoltage) to an access line connected to memory cells selected forsensing, and detecting whether one or more of the memory cells areactivated in response to the read voltage, such as by sensing a changein voltage levels or current levels of respective data lines connectedto the selected memory cells. For example, a read voltage having aparticular voltage level (e.g., a voltage level of the bottom of thethreshold voltage range 611 b) might be applied to the access lineconnected to the memory cells of the distribution of threshold voltages403, and those memory cells for which activation is detected might beassigned to the group of memory cells corresponding to the thresholdvoltage range 611 a. The voltage level of the read voltage might then beincreased (e.g., to a voltage level of the bottom of the thresholdvoltage range 611 c), and those additional memory cells for whichactivation is detected might be assigned to the group of memory cellscorresponding to the threshold voltage range 611 b. This process can berepeated until each of the respective groups of memory cellscorresponding to the threshold voltage ranges 611 a-611 h areidentified. Note that sensing with a read voltage at the top of thethreshold voltage range 611 h may be unnecessary where threshold voltagerange 611 h is sufficiently high that it can be presumed there are nomemory cells having threshold voltages above the threshold voltage range611 h. In such a case, each memory cell of the distribution of thresholdvoltages 403 that was not previously activated can be assigned to thegroup of memory cells corresponding to the threshold voltage range 611h.

Information indicative as to which group of memory cells a particularmemory cell corresponds might be stored in a latch, register or otherstorage media, e.g., other memory cells of the array of memory cells.Using the example of FIG. 6A, where the threshold voltage range of thedistribution of threshold voltages 403 is segmented into eight thresholdvoltage ranges 611, a three-bit latch or register could storeinformation indicative of the corresponding group for a particularmemory cell. Although the threshold voltage ranges 611 are depicted inFIG. 6A to have a combined width coinciding with the width of thedistribution of threshold voltages 403, it may be beneficial to havethreshold voltage ranges 611 extending beyond the expected width of thedistribution of threshold voltages 403 to mitigate the possibility of aselected memory cell having a threshold voltage of less than the lowestthreshold voltage range, e.g., threshold voltage range 611 a in thisexample, or greater than the highest threshold voltage range, e.g.,threshold voltage range 611 h in this example.

Each of the identified groups of memory cells may be associated with arepresentative threshold voltage, and thus a particular value of VgVt.For example, a representative threshold voltage of a group of memorycells might be a lower value of its corresponding threshold voltagerange 611, an upper value of its corresponding threshold voltage range611, an average of its corresponding threshold voltage range 611, etc.The value of VgVt for a particular group of memory cells can then bedetermined as the voltage level of the discovery programming pulse minusthe representative threshold voltage of that group of memory cells. Thisgroup of memory cells might then be programmed to an intended data stateusing a subsequent programming pulse (e.g., a subsequent programmingpulse having multiple decremented steps) having a voltage level (e.g.,of a plurality of voltage levels) determined from the value of VgVt forthat group of memory cells and a threshold voltage (e.g., an intendedthreshold voltage) corresponding to the intended data state. Forexample, the subsequent programming pulse might have a voltage levelcalculated from (e.g., the sum of) the value of VgVt and the thresholdvoltage (e.g., a lower threshold voltage) of the intended data state. Itis noted that the value of VgVt used for determining the voltage levelof the subsequent programming pulse may be the determined VgVt for thatgroup of memory cells or it may be a corrected value, e.g., calculatedfrom some defined function of a representative threshold voltage of theintended data state. For some embodiments, the representative thresholdvoltage of the intended data state, or intended range of thresholdvoltages, is a lower value (e.g., lower limit) of the intended datastate. This representative threshold voltage might, for example,correspond to a voltage level of a verify pulse used to verify if amemory cell has been sufficiently programmed to indicate the intendeddata state.

FIG. 6B depicts such programming of the group of memory cellscorresponding to the threshold voltage range 611 a. Followingapplication of the subsequent programming pulse, those memory cellscorresponding to the threshold voltage range 611 a might have theirthreshold voltages shifted to a threshold voltage range 613, which mightbe within an intended threshold voltage range, resulting in adistribution of threshold voltages 603 a. The shift in thresholdvoltages may be a linear shift, thus resulting in a distribution havinga same (e.g., similar) shape as that portion of the distribution ofthreshold voltages 403 corresponding to the threshold voltage range 611a. Memory cells connected to the selected access line and in groups notcorresponding to the threshold voltage range 611 a, or otherwise notselected for programming, might be inhibited from programming duringthis portion of the programming operation.

FIG. 6C depicts programming of the group of memory cells correspondingto the threshold voltage range 611 b. For programming of the group ofmemory cells corresponding to the threshold voltage range 611 b, thevoltage level of the subsequent programming pulse can be decreased. Forexample, the voltage level might be decreased by a value correspondingto (e.g., equal to) the VgVt for the group of memory cells correspondingto the threshold voltage range 611 a minus the VgVt for the group ofmemory cells corresponding to the threshold voltage range 611 b. Thisvalue might be equal to a width of the threshold voltage ranges 611.Alternatively, the decreased voltage level of the subsequent programmingpulse can be determined using values of VgVt calculated from the definedfunction of the particular threshold voltage of the intended data state.Regardless of how the voltage level of the subsequent programming pulseis determined, those memory cells corresponding to the threshold voltagerange 611 b might have their threshold voltages shifted to the thresholdvoltage range 613 as a result. The resulting distribution of thresholdvoltages 603 ab represents a sum of the portions of the distribution ofthreshold voltages 403 corresponding to the threshold voltage ranges 611a and 611 b. Memory cells connected to the selected access line and ingroups not corresponding to the threshold voltage range 611 b, orotherwise not selected for programming, might be inhibited fromprogramming during this portion of the programming operation.

FIG. 6D depicts such programming of the group of memory cellscorresponding to the threshold voltage range 611 c. For programming ofthe group of memory cells corresponding to the threshold voltage range611 c, the voltage level of the subsequent programming pulse can bedecreased. For example, the voltage level might be decreased by a valuecorresponding to (e.g., equal to) the VgVt for the group of memory cellscorresponding to the threshold voltage range 611 b minus the VgVt forthe group of memory cells corresponding to the threshold voltage range611 c. This value might be equal to a width of the threshold voltageranges 611. Alternatively, the decreased voltage level of the subsequentprogramming pulse can be determined using values of VgVt calculated fromthe defined function of the particular threshold voltage of the intendeddata state. Regardless of how the voltage level of the subsequentprogramming pulse is determined, those memory cells corresponding to thethreshold voltage range 611 c might have their threshold voltagesshifted to the threshold voltage range 613 as a result. The resultingdistribution of threshold voltages 603 abc represents a sum of theportions of the distribution of threshold voltages 403 corresponding tothe threshold voltage ranges 611 a, 611 b and 611 c. Memory cellsconnected to the selected access line and in groups not corresponding tothe threshold voltage range 611 c, or otherwise not selected forprogramming, might be inhibited from programming during this portion ofthe programming operation.

The method described with respect to FIGS. 6B-6D can be repeated foreach subsequent group of memory cells. Note that the slope of thedistribution of threshold voltages 603 abc may begin to decrease assubsequent groups of memory cells are programmed and added to thedistribution, and may approximate a flat line, in theory, after allgroups of memory cells are programmed. However, in view of naturalvariations, it may be expected that the resulting distribution ofthreshold voltages may more closely resemble a normal distributionextending beyond the threshold voltage range 613. Such variations mightbe mitigated by selecting the width of the threshold voltage range 613to be less than the width of the threshold voltage range of the intendeddata state. In addition, as discussed subsequently, an additionalprogramming pulse might be performed to narrow the resultingdistribution.

FIG. 7A depicts a flowchart of a method of operating a memory accordingto an embodiment, and may represent a programming operation of thememory. At 720, a programming pulse (e.g., discovery programming pulse)is applied to an access line selected for programming. The programmingpulse has a voltage level expected to cause a shift in thresholdvoltages of memory cells of the selected access line that are selectedfor programming. These selected memory cells are enabled for programmingduring application of this programming pulse, while other memory cellsof the selected access line may be inhibited from programming. Forexample, the data lines connected to the selected memory cells might bebiased to an enable voltage, e.g., 0V, while the data lines connected tothe remaining memory cells might be biased to an inhibit voltage, e.g.,Vcc.

At 722, the selected memory cells are assigned to respective groups ofmemory cells, each having a different range of threshold voltages. Forexample, the selected memory cells may be sensed to determine aparticular range of threshold voltages to which a particular memory cellcorresponds. This may be accomplished by applying a series of increasingvoltage levels to the selected access line, and determining which of theselected memory cells are activated in response to each of the appliedvoltage levels, as described with reference to FIG. 6A. At 724, valuesof VgVt are determined for each group of memory cells. For example, arepresentative threshold voltage corresponding to a particular group ofmemory cells might be subtracted from the voltage level of theprogramming pulse applied to the selected access line, as described withreference to FIG. 6A.

At 726, a subsequent programming pulse is applied to the selected accessline. At 728, the selected memory cells of a particular group of memorycells are enabled for programming, while the selected memory cells ofthe remaining groups of memory cells are inhibited from programming. Forexample, the group of memory cells having the highest value of VgVtmight be enabled for programming while the remaining groups of memorycells might be inhibited. The voltage level of the subsequentprogramming pulse might have a value expected to shift the thresholdvoltages of memory cells enabled for programming. For example, thevoltage level of the subsequent programming pulse might have a valueexpected to shift the threshold voltages of the group of memory cellshaving the highest value of VgVt to a range of threshold voltagescorresponding to an intended data state.

It is noted that while a particular voltage level might be determined tobe expected to shift the threshold voltages of a group of memory cellsto a range of threshold voltages corresponding to the intended datastate, the nature of integrated circuit devices may not permitapplication of this exact voltage level. For example, a memory devicemay be configured to generate some finite number of different voltagelevels for a programming pulse. As such, applying a selected voltagelevel (e.g., a nearest voltage level or next lower voltage level) of thefinite number of different voltage levels for application to theselected access line in response to a determination of the particularvoltage level expected to shift the threshold voltages of a group ofmemory cells to a range of threshold voltages corresponding to theintended data state, where that selected voltage level is also expectedto shift the threshold voltages of the group of memory cells to a rangeof threshold voltages corresponding to the intended data state, isequivalent to applying the determined particular voltage level.

At 730 a decision is made whether all of the groups of memory cells havebeen programmed using the subsequent programming pulse, e.g., whetherthe programming operation is complete. If yes, the method can end at732. If no, a next group of memory cells is selected at 734 and thevoltage level of the subsequent programming pulse is changed at 736. Thevoltage level of the subsequent programming pulse is changed to a valuecorresponding to the group of memory cells selected at 734. For example,the group of memory cells having the next lower value of VgVt might beselected, and the voltage level of the subsequent programming pulsemight be decreased by a value corresponding to the difference betweenthe VgVt (e.g., determined or corrected) of the prior group of memorycells and the VgVt (e.g., determined or corrected) of the next group ofmemory cells. For example, the voltage level of the subsequentprogramming pulse might be decreased to a value expected to shift thethreshold voltages of the selected group of memory cells to a range ofthreshold voltages corresponding to the intended data state. Note thatwhile the examples have used decreasing voltage levels of the subsequentprogramming pulse with decreasing values of VgVt, increasing voltagelevels for the subsequent programming pulse might be used, such as byfirst selecting the group of memory cells having the lowest value ofVgVt at 728, then selecting the group of memory cells having the nexthigher value of VgVt at 734. Other orders are also permissible. Notethat the prior group of memory cells might be inhibited from programmingprior to changing the voltage level of the subsequent programming pulseat 736.

In response to (e.g., after or with) changing the voltage level of thesubsequent programming pulse at 736, the selected memory cells of thenext group of memory cells are enabled for programming while theselected memory cells of the remaining groups of memory cells areinhibited from programming while the subsequent programming pulse is atthe changed voltage level. This process loop of 728 to 736 can berepeated until all groups of memory cells have their respective memorycells programmed at their respective voltage levels of the subsequentprogramming pulse.

Although the method described with reference to FIG. 7A utilized justone intended data state, multiple data states might be programmed in aparticular programming operation. This might be accomplished byrepeating the process of 728 to 730 for each data state, once for afirst intended data state using a first subsequent programming pulse,and once for a second intended data state using a second subsequentprogramming pulse, and so on. This might be performed in order ofincreasing or decreasing threshold voltages of the intended data states,for example. To extend the method of FIG. 7A, blocks 728 and 730 mightbe modified as depicted in the variation of FIG. 7B. For example, at728, those selected memory cells of a group of memory cells having aparticular intended data state are enabled 740 for programming at aparticular voltage level of the subsequent programming pulse. Thoseselected memory cells of that group of memory cells having a differentintended data state may be inhibited 742 from programming at theparticular voltage level of the subsequent programming pulse, while theselected memory cells of the remaining groups of memory cells areinhibited 744 from programming at the particular voltage level of thesubsequent programming pulse.

At 729, a decision is made whether the selected memory cells having theparticular intended data state for all of the groups have beenprogrammed using the subsequent programming pulse. If no, the processcontinues to 734. If yes, the process continues to 730, where thedecision is made whether all of the groups of memory cells have beenprogrammed for each intended data state using the subsequent programmingpulse. If yes, the method can end at 732. If no, a next intended datastate is selected at 733 and a next subsequent programming pulse isapplied to the selected access line at 726 to repeat the process of 728to 730 for the next intended data state. The voltage level (e.g.,initial voltage level) at 726 may be determined for each subsequentprogramming pulse as appropriate for the threshold voltage range oftheir respective intended data states. Note that the selected accessline might not be discharged between iterations of the subsequentprogramming pulse.

FIG. 8 conceptually depicts waveforms of voltage levels for variousnodes in performing a method such as described with reference to FIG.7A. The embodiment of FIG. 8 utilizes eight groups of memory cellscorresponding to eight threshold voltage ranges, such as described withreference to FIGS. 6A-6D. Sel WL corresponds to the voltage level of theselected access line (e.g., selected word line). BL0-BL7 each correspondto the voltage levels of the groups of data lines (e.g., bit lines)selectively connected to selected memory cells of a respective group ofmemory cells having different values of VgVt. BLunsel corresponds todata lines (e.g., bit lines) selectively connected to memory cells forwhich no change in data state is desired during the programmingoperation (i.e., unselected memory cells), or to memory cells connectedto the selected access line that are not addressed by the programmingoperation.

At time t0, BL0-BL7 and BLunsel might be raised to an inhibit voltage,e.g., Vcc, while Sel WL might be raised to an intermediate voltagelevel. The intermediate voltage level of Sel WL is often utilized duringprogramming operations to reduce the power needed to raise the voltagelevel of Sel WL to its intended voltage level. For example, raising SelWL to the intermediate voltage level, and then raising Sel WL to itsintended voltage level while raising unselected access lines (not shownin FIG. 8) to some pass voltage, may consume less power than simplyraising Sel WL to its intended voltage level in one step. It is notedthat a variety of schemes are known for increasing a voltage level of aselected access line to its intended voltage level and for operatingunselected access lines connected to a same string of memory cells asthe selected access line. However, details of such schemes are notimportant to understanding the concepts disclosed herein.

At time t1, Sel WL is raised to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL0 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 a) to a range of threshold voltagescorresponding to an intended data state. The voltage level of BL0 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL0might subsequently be returned to the inhibit voltage, e.g., at orbefore time t2.

At time t2, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL1 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 b) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL1 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL1might subsequently be returned to the inhibit voltage, e.g., at orbefore time t3.

At time t3, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL2 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 c) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL2 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL2might subsequently be returned to the inhibit voltage, e.g., at orbefore time t4.

At time t4, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL3 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 d) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL3 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL3might subsequently be returned to the inhibit voltage, e.g., at orbefore time t5.

At time t5, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL4 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 e) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL4 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL4might subsequently be returned to the inhibit voltage, e.g., at orbefore time t6.

At time t6, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL5 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 f) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL5 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL5might subsequently be returned to the inhibit voltage, e.g., at orbefore time t7.

At time t7, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL6 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 g) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL6 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL6might subsequently be returned to the inhibit voltage, e.g., at orbefore time t8.

At time t8, Sel WL is decremented to a voltage level expected to programmemory cells of the group of memory cells corresponding to BL7 (e.g.,those memory cells having threshold voltages corresponding to thethreshold voltage range 611 g) to a range of threshold voltagescorresponding to the intended data state. The voltage level of BL7 mightalso be changed to the enable voltage, e.g., 0V, to enable programmingof the corresponding group of memory cells. The voltage level of BL7might subsequently be returned to the inhibit voltage, e.g., at orbefore time t9. At time t10, Sel WL, BL0-BL7 and BLunsel might bedischarged with the programming operation complete.

As noted previously, the distribution of threshold voltages 403following the discovery programming pulse might encompass negativethreshold voltages. If reliable methods of sensing negative thresholdvoltages are not available within a particular memory device, thegrouping of memory cells into threshold voltage ranges might beperformed in more than one step. FIGS. 9A-9B depict an alternate methodof determining VgVt for memory cells of a distribution of thresholdvoltages including negative threshold voltages according to anembodiment.

As depicted in FIG. 9A, following application of the discoveryprogramming pulse, a portion of the resulting distribution of thresholdvoltages 403 lies below the 0V level of threshold voltage while aportion lies above. A first sensing of the memory cells corresponding tothe distribution of threshold voltages 403 is performed using a readvoltage of 0V to determine whether each memory cell has a thresholdvoltage of 0V or higher, or a threshold voltage less than 0V. Anadditional discovery programming pulse is then applied to the memorycells determined to have threshold voltages less than 0V to shift theirdistribution higher. For example, this additional programming pulsemight have a voltage level expected to shift the threshold voltages ofeach memory cell having a threshold voltage less than 0V to a thresholdvoltage of 0V or higher, such as represented by distribution ofthreshold voltages 403 ₁. Those memory cells determined to havethreshold voltages of 0V or higher following the first discoveryprogramming pulse might be represented by distribution of thresholdvoltages 403 ₂.

Following application of the additional discovery programming pulse, aplurality of groups of memory cells are identified. These groupsrepresent portions of the memory cells of the distribution of thresholdvoltages 403 ₁ having threshold voltages within respective thresholdvoltages ranges 611 a-611 d, and of the distribution of thresholdvoltages 403 ₂ having threshold voltages within respective thresholdvoltages ranges 611 a-611 d, as depicted in FIG. 9B. For example, thosememory cells within distribution of threshold voltages 403 ₁ and havinga threshold voltage within the threshold voltage range 611 a mightcorrespond to a first group of memory cells, those memory cells withindistribution of threshold voltages 403 ₁ and having a threshold voltagewithin the threshold voltage range 611 b might correspond to a secondgroup of memory cells, those memory cells within distribution ofthreshold voltages 403 ₁ and having a threshold voltage within thethreshold voltage range 611 c might correspond to a third group ofmemory cells, and those memory cells within distribution of thresholdvoltages 403 ₁ and having a threshold voltage within the thresholdvoltage range 611 d might correspond to a fourth group of memory cells,each having a respective VgVt determined using the voltage level of theadditional discovery programming pulse.

To continue with the example, those memory cells within distribution ofthreshold voltages 403 ₂ and having a threshold voltage within thethreshold voltage range 611 a might correspond to a fifth group ofmemory cells, those memory cells within distribution of thresholdvoltages 403 ₂ and having a threshold voltage within the thresholdvoltage range 611 b might correspond to a sixth group of memory cells,those memory cells within distribution of threshold voltages 403 ₂ andhaving a threshold voltage within the threshold voltage range 611 cmight correspond to a seventh group of memory cells, and those memorycells within distribution of threshold voltages 403 ₂ and having athreshold voltage within the threshold voltage range 611 d mightcorrespond to an eighth group of memory cells, each having a respectiveVgVt determined using the voltage level of the first discoveryprogramming pulse. Programming of memory cells of the identified groupsof memory cells using a stepped programming pulse may now be performed,such as described with reference to FIGS. 5-8.

FIG. 10 depicts a method of reducing a width of a distribution ofthreshold voltages for use with various embodiments. The distribution ofthreshold voltages 603 might represent the resulting distribution ofcompleting the programming of each group of memory cells, such asdescribed with reference to FIGS. 6A-6D. The distribution of thresholdvoltages 603 might have a width 613. To reduce this width, thusresulting in a more narrow distribution, a sensing of the memory cellscorresponding to the distribution of threshold voltages 603 could beperformed using a read voltage having a voltage level equal to thevoltage level occurring at 1010 to determine which memory cells havethreshold voltages in the threshold voltage range 1012 and which memorycells have threshold voltages in the threshold voltage range 1014. Anadditional programming pulse is then applied to the memory cellsdetermined to have threshold voltages in the threshold voltage range1012. For example, this additional programming pulse might have avoltage level expected to shift the threshold voltages of each memorycell having a threshold voltage in threshold voltage range 1012 to athreshold voltage in threshold voltage range 1014. The additionalprogramming pulse might be a stepped programming pulse such as describedwith reference to FIGS. 5-8.

CONCLUSION

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe embodiments will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the embodiments.

What is claimed is:
 1. A method of operating a memory, comprising:applying a programming pulse having a particular voltage level to aselected access line connected to a plurality of memory cells selectedfor programming during a programming operation; concurrently enablingfor programming each memory cell of the plurality of memory cellsselected for programming while applying the programming pulse; afterapplying the programming pulse having the particular voltage level tothe selected access line and concurrently enabling for programming eachmemory cell of the plurality of memory cells selected for programmingwhile applying the programming pulse, applying a plurality of readvoltages to the selected access line; determining which memory cells ofthe plurality of memory cells selected for programming are activated inresponse to the application of each respective read voltage of theplurality of read voltages; for each read voltage of the plurality ofread voltages, assigning those memory cells determined to be activatedin response to that read voltage, but not activated in response to anypreceding read voltage of the plurality of read voltages, to arespective group of memory cells of the plurality of groups of memorycells in response to that read voltage and respective intended datastates of those memory cells; for each read voltage of the plurality ofread voltages, determining a value of VgVt for those memory cellsdetermined to be activated in response to that read voltage, but notactivated in response to any preceding read voltage of the plurality ofread voltages; after applying the plurality of read voltages to theselected access line, applying a subsequent programming pulse having aplurality of different voltage levels to the selected access line; andfor each group of memory cells of a plurality of groups of memory cellsof the plurality of memory cells selected for programming, enabling thatgroup of memory cells for programming while the subsequent programmingpulse has a corresponding voltage level of the plurality of differentvoltage levels; wherein determining the value of VgVt for a particulargroup of memory cells of the plurality of groups of memory cellscomprises subtracting a representative threshold voltage of theparticular group of memory cells and a voltage level of the channels ofthe selected memory cells of the particular group of memory cells fromthe particular voltage level of the programming pulse.
 2. The method ofclaim 1, further comprising calculating a corrected value of VgVt forthe at least one memory cell in response to a function of therepresentative threshold voltage.
 3. The method of claim 1, wherein theprogramming pulse is a first programming pulse, and further comprisingapplying a second programming pulse, after the first programming pulseand prior to the subsequent programming pulse, having a voltage levelhigher than the particular voltage level of the first programming pulseto the selected access line while the memory cells of at least theparticular group of memory cells are enabled for programming, anddetermining that at least one memory cell of the particular group ofmemory cells has a negative threshold voltages prior to applying thesecond programming pulse.
 4. A method of operating a memory, comprising:applying a first programming pulse having a particular voltage level toa selected access line connected to a plurality of memory cells selectedfor programming during a programming operation; concurrently enablingfor programming each memory cell of the plurality of memory cellsselected for programming while applying the first programming pulse;determining which memory cells of the plurality of memory cells selectedfor programming have a negative threshold voltage; applying a secondprogramming pulse having a different voltage level, higher than theparticular voltage level, to the selected access line; concurrentlyenabling for programming each memory cell of the plurality of memorycells selected for programming that was determined to have a negativethreshold voltage while applying the second programming pulse, and whileconcurrently inhibiting from programming each remaining memory cell ofthe plurality of memory cells selected for programming while applyingthe second programming pulse; assigning each memory cell of theplurality of memory cells selected for programming to a respective groupof memory cells of a plurality of groups of memory cells in response toa sensed threshold voltage of that memory cell following the secondprogramming pulse; applying a third programming pulse having a pluralityof different voltage levels to the selected access line, wherein eachvoltage level of the plurality of different voltage levels occurs duringa respective portion of a duration of the third programming pulse; andfor each group of memory cells of the plurality of groups of memorycells of the plurality of memory cells selected for programming,enabling that group of memory cells for programming during therespective portion of the duration of the third programming pulse of acorresponding voltage level of the plurality of different voltagelevels; wherein memory cells of the plurality of memory cells selectedfor programming and having a particular intended data state are membersof more than one group of memory cells of the plurality of groups ofmemory cells; and wherein at least one group of memory cells of theplurality of groups of memory cells comprises a memory cell having theparticular intended data state and a memory cell having a differentintended data state.